In general, in the production processes of high-density semiconductor integrated circuits such as LSI, color filters for CCD (charge-coupled device) and LCD (liquid crystal display device), magnetic heads, etc., microfabrication utilizing the photolithographic technique using photomasks is performed.
In this microfabrication, a photomask, in which a light-shielding film made of a metal thin film such as a chromium film is generally formed on a light transmissive substrate made of quartz glass, aluminosilicate glass or the like by means of sputtering, vacuum deposition or the like to provide a photomask blank, wherein the light-shielding film is formed to have a predetermined pattern, is used.
A photomask is produced using such a photomask blank with the following processes: an exposure process in which a desired pattern exposure is applied to a resist film formed on the photomask blank; a development process in which, after the desired pattern exposure is applied to the resist film formed on the photomask blank, a developing solution is supplied thereto to dissolve portions of the resist film soluble in the developing solution, thereby forming a resist pattern; a etching process in which, using the obtained resist pattern as a mask, portions in which a light-shielding film is exposed with the resist pattern not formed are removed by etching, such as, wet etching using an etching solution consisting of a mixed aqueous solution of ceric ammonium nitrate and perchloric acid, and dry etching using chlorine gas, thereby forming a predetermined mask pattern on a light transmissive substrate; and a stripping/removing process in which the remaining resist pattern is stripped and removed.
In general, such a light-shielding film made of a chromium film or the like has a high optical reflectance, and a phenomenon, in which light reflected by a semiconductor substrate that is an exposed product passes through a projector lens and is reflected by a photomask to return to the semiconductor substrate, occurs. As a result, an undesired portion is irradiated with an exposure light. In order to prevent such a phenomenon, usually, an antireflection layer is formed on the surface of a light-shielding film of a photomask blank.
A photomask is washed at the time of production or use thereof. At the time of such washing, an acid such as sulfuric acid is often used (for example, see Japanese Laid-Open Patent Publication No. 2003-248298 (Patent Document 1)). However, when the photomask is washed with sulfuric acid, sulfuric acid or sulfate ion that remains after washing reacts with high-energy exposure light to be precipitated as ammonium sulfide, and this causes haze on the photomask. Recently, the exposure light to be used has been changed from a KrF excimer laser light (wavelength: 248 nm) to an ArF excimer laser light (wavelength: 193 nm). Due to such shortening of the wavelength, energy of the exposure light has been increased. As a result, generation of haze on the photomask due to washing with an acid such as sulfuric acid has been becoming a significant problem.
In order to prevent such haze caused by washing with an acid, recently, it has began to employ ozone wash using ozone water, UV ozone or the like at the time of production of a photomask blank or after pattern formation.
However, ozone wash dissolves or deteriorates a light-shielding film, and this may change optical properties (reflectance, etc.) of the light-shielding film or an antireflection film.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2003-248298